In the prior art, the Fischer-Tropsch process has been used for decades to assist in the formulation of hydrocarbons. In the last several years, this has become a concern given the escalating environmental concerns regarding poor fuel quality and pollution, together with the increasing costs of hydrocarbon exploration and refining. The major producers in this area have expanded the art significantly in this technological area with a number of patented advances and pending applications in the form of publications.
In the art, advances made in terms of the raw materials that have been progenitor materials for the Fischer-Tropsch process, have included, for example, coal-to-liquid (CTL), bio-to-liquid (BTL) and gas-to-liquid (GTL). One of the more particularly advantageous features of the gas-to-liquid (GTL) technology is the fact that it presents a possibility to formulate a higher value environmentally beneficial synthetic diesel product or syndiesel from stranded natural gas reserves, which would otherwise have not been commercially feasible to bring to market. As is generally known, the Fischer-Tropsch (FT) process converts hydrogen and carbon monoxide (commonly known as syngas) into liquid hydrocarbon fuels, examples of which include synthetic diesel, naphtha, kerosene, aviation or jet fuel and paraffinic wax. As a precursory step, the natural gas is thermally converted using heat and pressure in the presence of catalyst to produce a hydrogen rich syngas containing hydrogen and carbon monoxide. As a result of the Fischer-Tropsch technique, the synthetic fuels are very appealing from an environmental point of view, since they are paraffinic in nature and substantially devoid of contamination. This is particularly true in the case of the diesel fuel synthesis where the synthetic product has ideal properties for diesel engines, including extremely high cetane rating>70, negligible aromatics and sulphur content, in addition to enabling optimum combustion and virtually emission free operation. Synthetic diesel or syndiesel fuels significantly reduce nitrous oxide and particulate matter when compared with petroleum based diesel fuel.
U.S. Pat. No. 6,958,363 (Espinoza, et al.) teaches a process for synthesizing hydrocarbons where initially, a synthesis gas stream is formulated in a syngas generator. The synthesis gas stream comprises primarily hydrogen and carbon monoxide. The process involves catalytically converting the synthesis gas stream in a synthesis reaction to produce hydrocarbons and water followed by the generation of hydrogen-rich stream in the hydrogen generator. The process indicates that the hydrogen generator is separate from the syngas generator (supra) and that the hydrogen generator comprises either a process for converting hydrocarbons to olefins, a process for catalytically dehydrogenating hydrocarbons, or a process for refining petroleum, and a process for converting hydrocarbons to carbon filaments. The final step in the process in its broadest sense, involves consumption of hydrogen from the hydrogen-rich stream produced in one or more processes that result and increase value of the hydrocarbons or the productivity of the conversion of the hydrocarbons from the earlier second mentioned step.
Although a useful process, it is evident from the disclosure of Espinoza et al. that there is a clear intent to create olefins such as ethylene and propylene for petrochemical use, and aromatics for gasoline production. Additionally, there is a reforming step indicated to include the reformation of naphtha feedstock to generate a net surplus hydrogen by-product which is then recombined into the process. The naphtha is subsequently converted to aromatics for high octane gasoline blend stock.
U.S. Pat. No. 7,214,720 (Bayle et al.) discloses the production of liquid fuels by a concatenation of processes for treatment of a hydrocarbon feedstock. It is indicated in the disclosure that the liquid fuels begin with the organic material, typically biomass as a solid feedstock. The process involves a stage for the gasification of the solid feedstock, a stage for purification of synthesis gas and subsequently a stage for transformation of the synthesis gas into a liquid fuel.
This reference indicates in column 2 the essence of the technology:
“A process was found for the production of liquid fuels starting from a solid feedstock that contains the organic material in which:
                a) The solid feedstock is subjected to a gasification stage so as to convert said feedstock into synthesis gas comprising carbon monoxide and hydrogen,        b) the synthesis gas that is obtained in stage a) is subjected to a purification treatment that comprises an adjustment for increasing the molar ratio of hydrogen to carbon monoxide, H2/CO, up to a predetermined value, preferably between 1.8 and 2.2,        c) the purified synthesis gas that is obtained in stage b) is subjected to a conversion stage that comprises the implementation of a Fischer-Tropsch-type synthesis so as to convert said synthesis gas into a liquid effluent and a gaseous effluent,        d) the liquid effluent that is obtained in stage c) is fractionated so as to obtain at least two fractions that are selected from the group that consists of: a gaseous fraction, a naphtha fraction, a kerosene fraction, and a gas oil fraction, and        e) at least a portion of the naphtha fraction is recycled in gasification stage.”        
The naphtha recycle stream that is generated in this process is introduced into the gasification stage. To introduce the naphtha to the gasification stage as taught in Bayle et al., is to modify the H2/CO ratio in the gasification stage using an oxidizing agent such as water vapour and gaseous hydrocarbon feedstocks such as natural gas with the recycled naphtha, while maximizing the mass rate of carbon monoxide and maintain sufficient temperature above 1000° C. to 1500° C. in the gasification stage to maximize the conversion of tars and light hydrocarbons.
U.S. Pat. No. 6,696,501 (Schanke et al.) entitled Optimum Integration Process for Fischer-Tropsch Synthesis and Syngas Production discloses a process for the conversion of natural gas or other fossil fuels to higher hydrocarbons. In the process disclosed therein the natural gas or the fossil fuels is reacted with steam and oxygenic gas in a reforming zone to produce synthesis gas which primarily contains hydrogen, carbon monoxide and carbon dioxide. The synthesis gas is then passed into a Fischer-Tropsch reactor to produce a crude synthesis containing lower hydrocarbons, water and non-converted synthesis gas. Subsequently, the crude synthesis stream is separated in a recovery zone into a crude product stream containing heavier hydrocarbons, a water stream and a tail gas stream containing the remaining constituents. It is also taught that the tail gas stream is reformed in a separate steam reformer with steam and natural gas and then the sole reformed tail gas is introduced into the gas stream before being fed into the Fischer-Tropsch reactor.
In this reference, a high carbon dioxide stream is recycled back to an ATR in order to maximize the efficiency of the carbon in the process. It is further taught that the primary purpose of reforming and recycling the tail gas is to steam reform the lower hydrocarbons to carbon monoxide and hydrogen and as there is little in the way of light hydrocarbons, adding natural gas will therefore increase the carbon efficiency. In the Schanke et al. reference, the patentees primarily focused on the production of the high carbon content syngas in a GTL environment using an ATR as crude synthesis stream and reforming the synthesis tail gas in an SMR with natural gas addition to create optimum conditions that feed to the Fischer-Tropsch reactor.
In respect of other progress that has been made in this field of technology, the art is replete with significant advances in, not only gasification of solid carbon feeds, but also methodology for the preparation of syngas, management of hydrogen and carbon monoxide in a GTL plant, the Fischer-Tropsch reactors management of hydrogen, and the conversion of biomass feedstock into hydrocarbon liquid transportation fuels, inter alia. The following is a representative list of other such references. This includes: U.S. Pat. Nos. 7,776,114; 6,765,025; 6,512,018; 6,147,126; 6,133,328; 7,855,235; 7,846,979; 6,147,126; 7,004,985; 6,048,449; 7,208,530; 6,730,285; 6,872,753, as well as United States Patent Application Publication Nos. US2010/0113624; US2004/0181313; US2010/0036181; US2010/0216898; US2008/0021122; US 2008/0115415; and US 2010/0000153.
U.S. Pat. No. 7,168,265 discloses an integrated process for producing LNG and GTL products, wherein a CO2-containing natural gas feed to an LNG production zone is first pre-treated to separate at least a portion of the CO2 therefrom, and the resulting CO2 stream obtained thereby is then directed to a GTL production zone and utilized to make GTL products that include methanol and/or methanol derivatives.
Applicant's co-pending application Ser. No. 13/228,042 provides a process for synthesizing hydrocarbons, comprising: a) formulating a hydrogen rich stream with a syngas generator; b) catalytically converting said stream to produce hydrocarbons, containing at least naphtha; c) recycling at least a portion of said naphtha to said syngas generator to form an enhanced hydrogen rich stream; and d) re-circulating said enhanced hydrogen rich stream from step (c) for conversion in step (b) to enhance the synthesis of hydrocarbons.
Although the processes disclosed in U.S. Ser. No. 13/228,042 allow for the conversion of greater than 65% of all carbon in the feed streams to hydrocarbon products, and the process disclosed in U.S. Pat. No. 7,168,265 allow for co-production of methanol, there remains a need for technology that provides for an optimized conversion of the unconverted process CO2 and other by-products of the hydrocarbon production process to commercially valuable co-products such that 100% of all the carbon in captured CO2 by-product streams can be converted to valuable commercial co-products.
As part of the further advancements set forth herein, there are provided processes for the optimized production of commercially useful co-products from the by-products of the hydrocarbon synthesis process. These processes can be integrated within hydrocarbon synthesis systems as described, for example, in co-pending U.S. Ser. No. 13/228,042.